This was not my first major modification project (my first was an old Presonus MP4 headphone amp, AMAZING, which I might write about another time), but it was significant insofar as these are my studio monitors, and I basically wasn’t using them. They just weren’t clear enough. I had tried using Sonarworks Reference 4, which delivered big changes for sure, but I still found what was coming out muddy and incoherent. I primarily work with classical and acoustic stuff, so quick, snappy fidelity is all I care about. I know the Rokits are favorites of EDM folks, and I don’t really care about that stuff, but these were my first monitors and I didn’t know any better…
Anyway, I’ve been interested in audio modification for a while, and the arguments that swirl around them––op amp rolling is for dummies, capacitors don’t have a sound you’re just a phool, etc etc. I have a Behringer ADA8200 modded by Revive Audio, and I used to have an RME Babyface Pro modded by Black Lion Audio––the former sounds fantastic, the latter… well, the changes were extremely subtle, even if they were audible. Anyway, these sorts of modifications are quite popular, and I love the idea––rather than constantly buying new stuff, buy used stuff on its way to the garbage bin, pimp it out, and end up with a device that rivals high-end gear (better yet if you can do it yourself––you’ll pay a quarter what it costs to get someone else to do it). It makes complete sense to me that a company with a profit margin, like KRK or Presonus or M-Audio or even RME, would cut corners where they think it’ll be least noticed by 95% of folks in most normal-use situations (i.e., a singer-songwriter at home on their bed belting into some crappy Chinese mic). I don’t think it’s wrong to “tune” your equipment to sound the way you want.
To me the mindset is very much like what people think about in the instrumental world. I grew up with a Yamaha G2 grand piano, rather petit, about 5’7″ or so. Yamaha makes spectacularly reliable actions, but the sound usually leaves something to be desired. The hammers were rock solid (not in a good way), so when I was in college we had a piano technician replace them with a set custom made for the piano by Ari Issac in Toronto. The new set completely transformed the sound––there was color, finally, shading, resonance, a dynamic range (especially soft)––and over a decade later those hammers continue to break in, revealing new sounds I never thought I’d hear from such a small instrument. Recently I had the bass strings measured for replacements. Yamaha is not known for making great bass strings––they sound a bit like surgical tubing drawn taut. I haven’t yet heard the results, but I will when I return home to visit my family for the holidays. Anyway, modification is a very musical thing to do, so I don’t think people who are interested in modifying their audio equipment are somehow phoolish to think they can eke out every last drop of good sound. Your milage may vary, and that’s ultimately all that matters.
Okay, if you pay $50 for a capacitor made with raw sheep’s milk and gold dust from an ancient Andean prince, you are a phool. I’ll admit that much.
Anyway, I wasn’t using these Rokits much, plus the warrantee was out. What could go wrong?
Nothing wrong with them. They work just fine. This is the internal amp I was going to modify. The actual amplifiers are two integrated circuits (one for tweeter, one for woofer) pasted to the heat sink in back, the rest of this circuit is power supply, input buffer, and crossover. Power supply in the back (large capacitors and rectifier), input section in the front right using bipolar caps and two NJM4580s, crossover section in the middle front left with three quad TL072s and crossover filters (green mylar caps). The yellow caps up front appeared to be for the “room adjustment” filters on the back, and I never use those, so I figured I’d leave them alone.
Only way to remove the amplifier board was to desolder it from the input PCB. Definitely ripped a pad here, but it wasn’t a problem to repair later (just used a jumper or something).
[There’s also a “mystery circuit,” seen just to the top left of the input board that appears to supply 5V for a digital… something. Wasn’t connected to anything, so I wonder if KRK has another line of bluetooth products that use this amplifier board? I couldn’t find any. I left it alone.]
The electrolytic caps are all easy to replace (they all have capacitance and voltage ratings written on the case), but the mylar caps are another matter. They didn’t seem to have much of anything written on them, plus everything was bathed in this disgusting sticky glue which seemed to obscure any writing that may have been there. I used a capacitance meter to measure each cap after I removed it.
(sorry for my chicken scratch)
Couldn’t be exactly sure what the value was of each cap (they have a tolerance that usually is within 20% of the stated value), but rounding to the closest available value on Mouser or DigiKey usually revealed what they must have been.
Another view of the mylar caps (green).
To remove the op amps I very gently grasped them with a pair of pliers and used a hot air rework station to melt the solder. There’s no need to pull, the chips are so light, they basically float up off the pads the moment the solder melts (which usually takes about 10-20 seconds).
Clean board (all the SMD resistors, diodes, capacitors are left in place, of course):
Now had to remove all that glue:
Added extra supply rail decoupling, 0.1uF ceramic capacitors from each power rail to ground. Seemed there was one lonely ceramic cap between the rails for both input opamps, extra won’t hurt, and would certainly help prevent stability problems when I put in the faster audio-grade opamps (used OPA1642 for the dual opamps and 1644 for quad––FET input opamps, like the OPA164x are more likely than bipolar types to be stable in dubious situations, like this).
Replaced all polarized electrolytic caps with Panasonic FR, and bipolars with Panasonic SU. I kept all the values the same, although, come to think of it now, I might have done well to at least double all the signal path caps, especially because I bypassed every one with a Polypropylene film capacitor (also Panasonic, though I can’t remember which type). Mylar crossover caps were also replaced with Panasonic Polypropylene. A couple values I couldn’t find, so I used a Vishay polypropylene cap.
So I plugged it in and… no smoke! So how does it sound? I placed both speakers side by side (as pictured at the top of the page), one modified and the other stock, plugged them into the two outputs of my interface and routed an identical mono mix to both (I think it was of Mitsuko Uchida playing a Mozart concerto). “Panning” back and forth between the channels, the difference was NOT subtle. One gave the impression of having a wet towel wrapped around your head, the other didn’t and sounded like music. In fact, more than that, in the modded speaker I felt I could perceive a depth field even in the mono signal, like distance front-to-back between instruments on the stage. That surprised me. So hells yeah, it was worth it. I modded the other one promptly…
These are the 5-inch speakers, so they still have a limited low end. I’m not sure increasing signal path cap sizes would have helped that much. I understand there is a limiter circuit built into this amplifier somewhere that could be disconnected. Again, I don’t listen to EDM or anything with super heavy bass peaks that could blow out the voice coils, so I’m not worried about damaging the speakers, plus I’ve heard of folks doing this in the Yamaha HS8s to great effect. Finding it would be a matter of poking around with an oscilloscope, so maybe I’ll do this at some point.
So how do they compare to other more expensive speakers? I don’t know, I don’t own anything else, haven’t done a side-by-side with someone who does. I’m gonna go out on a limb and say they’re not Genelecs, but they also were $300 for the pair, plus $50 in parts, as opposed to $1500 for a pair. They’re certainly better, much more usable, more transparent and revealing, and that’s all I needed. Still, I’m not working in a particularly well-treated listening space, so these modifications can only go so far, but the initial tests revealed an obvious improvement.